Was Venus Once A Habitable World Like Earth?

To date, much of our search for past or present life elsewhere in the Solar System has focused on Mars. That’s not without reason; Mars is a rocky planet like Earth, has a thin atmosphere, and we’re pretty certain it once had vast seas of water on its surface. But maybe we should be considering another planet, too: Venus.

Venus today is anything but habitable. Its surface temperature is hot enough to melt lead, its atmosphere is a mixture of poisonous sulfuric acid and carbon dioxide, and it has no water. However, mounting evidence is suggesting this was not always the case.

A new study published in Geophysical Research Letters (pre-print on arXiv) from a team of US scientists modeled what Venus may once have looked like. The researchers created four models for the surface based on how fast it may have rotated (today it rotates once every 243 days) and how much sunlight it received. And, they suggest, in some scenarios, it could have supported a habitable environment anywhere between 2.9 billion and 715 million years ago, with oceans and even snow on the surface, and a temperature more similar to modern Earth.

This is not the first study to suggest that Venus might once have been habitable, but it provides a new insight into what Venus may have looked like in its past. A runaway greenhouse effect, where the temperature of the planet increased rapidly as its atmosphere got thicker and trapped more heat, ultimately led to the hellish world we see today. But is there a chance life may once have existed there?

“Present-day Venus is an inhospitable place with surface temperatures approaching 750K and an atmosphere over 90 times as thick as present day Earth's,” the study notes. “Billions of years ago the picture may have been very different.”

At the moment we have no direct evidence for water once being on the surface of Venus. On Mars we can see tantalizing evidence of ancient shorelines, and it’s possible we might be able to discover some on Venus, too, although much of the Venus we see today has been reshaped by volcanic activity about 700 million years ago, hiding evidence of an ancient ocean. But if it had water, and a habitable environment, then it’s not impossible it had life too.

“Both planets [Earth and Venus] probably enjoyed warm liquid water oceans in contact with rock and with organic molecules undergoing chemical evolution in those oceans,” study co-author David Grinspoon from the Planetary Science Institute in Arizona told New Scientist. “As far as we understand at present, those are the requirements for the origin of life.”

Given that Venus may have remained habitable for at most 2 billion years, it’s unclear if complex life could have arisen in that time. On Earth, for comparison, it took almost 3 billion years for complex life to take shape. But the idea is an interesting one.

Much more research is needed before we can come to a conclusion. But who knows – what is now regarded as Earth’s hellish evil twin may once have been much more friendly.